My computer is an Iqon Technologies desktop. It only has 258mb of Ram and is pretty slow so I want to add more memory. I ran the Crucial.com system scanner which came up with the following results:

Motherboard: GMate 8S651MP-RZ
CPU: Intel Celeron 2.66GHz model 3, stepping 4
CPU speed 2680 MHz
Maximum memory capacity: 2048mb
currently installed memory: 256mb

The current memory is DDR 333MHz CL2-5 PC2700U-25330.

I bought a 512mb memory stick made by PNY Technologies DDR 400MHz PC3200. According to Crucial this should be compatible with my system.

The first time I used the PC after fitting everything was fine, the computer's performance was much, much better. But the second time I turned it on it had stopped working. Although the computer will boot up, once it gets to the desktop it stops working. I can move the pointer AROUND but I can't select anything. Sometimes the pointer moves jerkily. When I REMOVED the new memory the computer was fine again. I tried the new memory in place of the old and removed the old altogether and the computer did the same thing, it would boot up to the desktop but then wouldn't work anymore. I thought this might be a problem with the memory stick and took it back to the shop who changed it for another new one but when I fitted that I had the same problem again. The PC is fine with the memory removed.

Does anybody have any ideas what the problem might be and how to fix it if possible?
You need to find if there is a relationship to the slots being used and the memory problem. Sometimes RAM is just bad and needs to be exchanged. Luckily Crucial has a lifetime warranty, so this is not a problem.

I would download the free RAM diagnostic at www.memtest86.com and try that with ONLY the new one. It creates a bootable CD or diskette, so Windows is not required.

Details about ALL of the characteristics of the RAM would be helpful. I have never heard of a GMate motherboard and cerftainly did not see it even mentioned on the crucial site. It could have problems as well. Im noticed that your current memory is DDR 333, and you got DDR 400 from corsair,sure it might work on you system. But I do not believe you can mix the two. You need to reset the memory to 400 in your bios/cmos setup. Depending on your bios Im sure there is a section where you can adjust the memory SETTINGS.I should also mention that sometimes different memory companies stick dont always work together,even though they should, such as when one is standard mem. and the other is non-standard(low density vs standard).That happened to me on older system using pc133,just a thought. I really believe the problem is that the settings on your bios/cmos setup are still set for your 333 and will require this to be changed in setup.What is your bios type ami? award? ??Suggest you go to your bios manufac. and check their support for setup help or check your motherboard's web page for support.SOMETIMES you can find the answer there,just a suggestion,if you havent done this already.I went online to see if I could find answers for the listed motherboard. Was unable to find a GMat board in any search engine, nor could I find it at the company you list as the pc's maker. They only show newer models and have no lists of any archieves, but was able to find the NUMBERS for a model gigabyte makes that matches you motherboard. Suspect your motherboard is a gigabyte or a copy of one . Anyway it shows the fastest speed for that to be 333ddr, so if you dont find a setup to change speed to 400 then I would swap the ram you got for 333 and it should work,better YET might consider going to pricewatch for possible better price assuming you can get money back,if not and can only get credit the I would go for 1 gig ram as that will show a SIGNIFACANT increase in speed over512.This is from my experience.I went from 256 to 512 some increase, but when I installed 2 512's to give 1 gig it really showed improvement.Hope these Ideas help.400 is backward compatible so running with 333 is not an issue, assuming the other characteristics are not a dealbreaker. The entire memory will run at the slowest speed of any single chip (333).